This invention relates generally to a drill feed control system and more particularly to an automatic variable control for setting the bias of an overcenter valve.
A drill feed force control system for a hydraulic drilling machine is used to limit the force applied to the material being drilled as the drilling device is advancing. The control system must be adjustable so that the drilling force may be altered depending upon the material which is being drilled. The control means must be able to accommodate drill strings of different weights, while allowing drilling force to remain approximately the same.
Drill feed advance is generally accomplished by providing the appropriate oil supply and return to a hydraulic motor or cylinder attached to a mechanical drill feed apparatus. Drill feed force control is generally accomplished by modifying the hydraulic pressures in the drive and the return oil conduits connected to the feed cylinders or motors.
Increasing return oil pressure biases the return chamber of the feed cylinder which limits the application of the feed force. This return bias is necessary to restrict the drill string from advancing on its own due to the influence of its own weight. The return bias is often referred to as "holdback". Increasing the pressure to the drive chamber adds bias to the drill string in the drive direction and acts to counteract the holdback force.
The amount of drive force and holdback force required for a specific drilling operation follow two basic rules. First, there must be enough holdback force available to hold up the weight of the drill string whenever the drill feed force control is at its minimum value. Since the drill weight changes as drill lengths are added and removed, the holdback force must be adjusted accordingly. Alternatively, the holdback device may be set to a fixed setting high enough to hold back the total anticipated string weight and the drive force adjusted to produce the desired drill feed force. The second rule is that the drill feed force must be predictable and repeatable when drilling. This is true especially in down hole hammer drilling operations where improper drill feed forces can result in damage to the drill components.
Drill machines in use today typically require one or two adjustable pressure controls so that the drill operator can control the drive and the holdback bias according to any one of the following three methods. The first method is to set the minimum holdback pressure at a fixed level sufficient to hold back the entire maximum drill string weight and adjust drive pressure to achieve the desired drill feed force. The second method is to set the maximum drive pressure at its minimum level and adjust the holdback pressure to achieve the desired drill feed force using only the available weight of the drill string to apply the drill feed force. The third method involves adjusting both the drive and holdback pressures to suit the immediate requirement for the drill feed force.
In all three methods, using the prior art feed control systems, the drill operator must know the relation between the drive force, holdback force and drill weight to accurately maintain the desired drill feed force. If the drill operator does not know this relationship the drill force cannot be predicted.
Even though experience in drilling in a particular area, gauges which indicate torque applied to the drill head and even the drilling sound of the drill head produced during the drilling action can provide indications as to the drill feed force, these methods are not precise and depend upon operator expertise.
The foregoing illustrates limitations known to exist in present drill feed force control systems used in drilling systems. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.